Continuous combustion systems such as gas turbine engines are used in a variety of industries. These industries include transportation, electric power generation, and process industries. During operation, the continuous combustion system produces energy by combusting fuels such as propane, natural gas, diesel, kerosene, or jet fuel. One of the byproducts of the combustion process is emission of pollutants into the atmosphere. The levels of pollutant emissions are regulated by government agencies. Despite significant reductions in the quantity of environmentally harmful gases emitted into the atmosphere, emission levels of gases such as NOx, CO, CO2 and hydrocarbon (HC) are regulated by the government to increasingly lower levels and in an ever increasing number of industries.
Industry developed various methods to reduce emission levels. One method for gaseous fueled turbines is lean premix combustion. In lean premix combustion, the ratio between fuel and air is kept low (lean) and the fuel is premixed with air before the combustion process. The temperature is then kept low enough to avoid formation of nitrous oxides (which occurs primarily at temperatures above 1850 K). The premixing also decreases the possibility of localized fuel rich areas where carbon monoxides and unburnt hydrocarbons are not fully oxidized.
One of the more difficult challenges facing manufacturers of lean premix gas turbines and other continuous combustion systems is the phenomenon of combustion instability. Combustion instability is the result of unsteady heat release of the burning fuel and can produce destructive pressure oscillations or acoustic oscillations. In lean premix gas turbines, combustion instability can occur when the air-fuel ratio is near the lean flammability limit, which is where turbine emissions are minimized and efficiency is maximized. In general, the air/fuel ratio of the premixed fuel flow should be as lean as possible to minimize combustion temperatures and reduce emissions. However, if the air/fuel ratio is too lean, the flame will become unstable and create pressure fluctuations. The typical manifestation of combustion instability is the fluctuation of combustion pressure sometimes occurring as low as +/−1 psi at frequencies ranging from a few hertz to tens of kHz. Depending on the magnitude and frequency, this oscillation can create an audible noise which is sometimes objectionable, but a much more serious effect can be catastrophic failure of turbine components due to high cycle fatigue. The most severe oscillations are those that excite the natural frequencies of the mechanical components in the combustion region, which greatly increases the magnitude of the mechanical stress.
Most continuous combustion systems are commissioned in the field with sufficient safety margin to avoid entering an operating regime where combustion instabilities can occur. However, as components wear out or fuel composition changes, the combustion process can still become unstable.